Larch needle blight
- French disease name: Rouge du mélèze
- Other disease names: Hypodermella needle cast, larch needle cast
- Pathogen name: Hypodermella laricis Tub.
- Kingdom: Fungi
- Phylum: Ascomycota
- Class: Leotiomycetes
- Order: Rhytismatales
- Family: Rhytismataceae
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Partial list of synonyms:
- Leptothyrella laricis Dearn
General information and importance
Hypodermella laricis, causal agent of larch needle blight, is pathogenic to all larch (Larix) species. In western Canada, the primary host is western larch (L. occidentalis), whereas in eastern Canada it is tamarack (L. laricina). Larch needle blight causes browning and death of needles, which remain attached to the spur shoots throughout the winter and continue to provide a source of inoculum in the tree canopy for the next generation of needles in the subsequent spring. Healthy needles would normally drop in the fall because larches are deciduous. The disease is also capable of killing spur shoots after repeated infections.
Distribution and hosts
The distribution of larch needle blight, which is endemic in North America, follows the natural distribution of its larch hosts. It occurs on western larch in southeastern British Columbia and Alberta and in the western United States. It also occurs on tamarack in Ontario, Quebec, the maritime provinces, and the eastern United States. It has also been reported on subalpine larch (L. lyallii) in British Columbia. Larch needle blight affects trees of all ages. It was introduced to Europe more than 100 years ago, where it is now common in alpine larch forests, and also on larch in Siberia.
Tree parts affected
Needles and spur shoots (short lateral branch shoots bearing clusters of needles) are affected. Long shoot death after repeated severe infections has been reported from Oregon, but not observed farther north in British Columbia.
Symptoms and signs
Larch needle blight initially causes yellowing of needles, followed by browning and death. Severe infections involving the entire crown can occur in younger understory trees. In larger trees, the lower two-thirds of the crown is most affected. Infected needles wither and die on the spur shoots where they may remain attached for up to two years. An inconspicuous conidial state of H. laricis develops shortly after infection, consisting of tiny numerous black pycnidia measuring 120–300 micrometres × 80–120 micrometres. Conidia produced in the pycnidia are hyaline (colourless), elongated pear–shaped, and 4–5 micrometres × 1 micrometre in size. Following conidial production is the development of an elongated ascocarp with a longitudinal slit, called a hysterothecium, which usually forms from the base of the needles. Hysterothecia are black, elliptical, subcuticular (forming under the cuticle of the needle), occurring in a continuous row along the long axis of the needle, averaging 0.5–0.8 millimetres × 0.2–0.3 millimetres in size. Beneath the dark covering layer of the hysterothecium is the hymenium, consisting of a layer of club-shaped asci interspersed with paraphyses (sterile cells). Asci are four-spored, pointed at the tip, and measure 80–112 micrometres × 20–24 micrometres. Ascospores are hyaline, single-celled, club-shaped, tapering to a pointed base, measure 70–105 micrometres × 6 micrometres, and are coated with a gelatinous sheath 5 micrometres thick. Paraphyses are slightly shorter than the asci, filiform, and slightly swollen at the tips.
Weather conditions (see Disease Cycle) that promote infection by H. laricis also increase the incidence of Rhabdocline laricis, another foliar disease of larch. However, R. laricis infections result in casting of larch needles, and do not produce visible fruiting bodies on the infected needles. Microscopic examination is required to confirm its presence.
Disease cycle
Clusters of withered dead larch needles bearing fruiting bodies remain attached to larch spur shoots after being killed, becoming the primary source of inoculum the following spring. Hypodermella laricis produces indole-3-acetic acid, a plant hormone that prevents the normal needle detachment process from occurring. The dead attached foliage can continue to bear new fruiting bodies for up to two years. In early spring, asci and ascospores in the hysterothecia mature, and they release their spores around the time larch buds are beginning to open and the new foliage is starting to expand. During rainy periods the contents of the hysterothecia swell, causing them to split lengthwise, exposing the hymenial layer beneath. The ascospores are then forcibly ejected from the asci and are further spread by rain splash. If the larch foliage is still immature (the protective waxy cuticle is not fully developed), the ascospores germinate and infect the new needles at their bases. Infected foliage begins to first turn yellowish orange, then brown about 4 weeks after infection, and fruiting bodies start to develop in late summer. Although hysterothecia are visible on the needles in the late summer, fall and winter, the hymenial layer does not mature and produce spores until the following spring. Shortly after infection, the fungus also produces a conidial stage, but the function of the conidia in the life cycle is unknown.
Damage
Damage due to high levels of infection and defoliation include growth loss, terminal shoot death, and, in the case of seedlings, death of the entire plant. Epidemics of larch needle blight occur at least once a decade in western North America. High levels of inoculum from the previous year, and wet spring weather coinciding with bud break and hysterothecium maturation create the right conditions to cause epidemics. During epidemic outbreaks the browning of needles can be widespread, striking, and sudden, but such occurrences are infrequent and not significant to mature tree health in the long term.
Needle loss after one season of infection can weaken the spur shoots, causing fewer, smaller, and less vigorous needle production the following year. Previously infected spur shoots are also slower to produce needles in the spring compared with uninfected spur shoots. Successive years of infection may result in spur shoot mortality. Dieback can also occur in long shoots (juvenile growth) that have been severely infected.
Larch needle blight is not known to cause mature tree mortality in Canada.
Prevention and management
Individual larch trees show considerable variation in disease resistance, so selection for resistance to the disease in precommercial thinning of forests would enhance overall forest health. Selection of resistant seed trees is also possible.
Pest management strategies for a particular pest vary depending on several factors. These include:
- the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
- the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
- an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
- how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
- the value of the host(s) versus the costs of pest management approaches; and
- consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.
Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.
Photos
Discoloured foliage of western larch affected by Hypodermella laricis
Bob Erickson
Hysterothecia of Hypodermella laricis on dead western larch needles
John Hopkins
Dead needles infected by Hypodermella laricis retained on western larch
Leo Unger
Selected references
Allen, E.; Morrison, D.; Wallis, G. 1996. Common tree diseases of British Columbia. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. 178 p.
Cohen, L. 1967. The pathology of Hypodermella laricis on larch, Larix occidentalis. American Journal of Botany 54(1): 118–124. https://doi.org/10.1002/j.1537-2197.1967.tb06898.x
Funk, A. 1985. Foliar fungi of western trees. Canadian Forestry Service, Pacific Forest Research Centre. Victoria, British Columbia. BC-X-265. 159 p.
Garbutt, R.W. 1985. Foliage diseases of western larch in British Columbia. Pacific Forest Research Centre, Canadian Forestry Service. Victoria, British Columbia. Forest Pest Leaflet 71. 4 p.
Hagle, S.K. 2004. Larch needle blight ecology and management. Chapter 15.2. Forest insect and disease management guide for the northern and central Rocky Mountains. United States Department of Agriculture, Forest Service, Northern Region, State and Private Forestry. 5 p https://dnrc.mt.gov/_docs/forestry/Forestry_Assistance/Forest_Pests/Needle-Diseases-Larch-Needle-Cast-Management-Guide-PDF-.pdf [Accessed September 2024].
Jalkanen, R. 2016. Synthesis and new observations on needle pathogens of larch in northern Finland. Forests 7(1): 25. https://doi.org/10.3390/f7010025
Leaphart, C.D.; Denton, R.E. 1961. Needle discolorations of western larch. United States Department of Agriculture Forest Service, Forest Service. Forest Pest Leaflet 61. 7 p. https://archive.org/details/CAT31114392/page/2/mode/2up [Accessed September 2024].
Scharpf, R.F. (technical coordinator). 1993. Diseases of Pacific coast conifers (revised). United States Department of Agriculture, Forest Service. Washington, D.C. Agriculture Handbook 521. 199 p. https://www.govinfo.gov/content/pkg/GOVPUB-A-PURL-gpo29040/pdf/GOVPUB-A-PURL-gpo29040.pdf [Accessed September 2024].
Sinclair, W.A.; Lyon, H.H. 2005. Diseases of trees and shrubs. Second edition. Comstock Publishing Associates, Cornell University Press. Ithaca, New York. 660 p.